Secondary batteries are becoming widely used as power supplies for vehicles and household appliances, not just as power supplies for portable devices such as mobile phones, digital cameras and laptop computers. From among the different kinds of secondary batteries, lithium ion secondary batteries, which have a high-energy density and are lightweight, are energy storage devices that have become essential in daily life. A laminated secondary battery has a battery electrode assembly having a structure in which positive electrode sheets and negative electrode sheets are alternately laminated with separators interposed therebetween. This battery electrode assembly and electrolyte are contained in an exterior container.
As the exterior container, there is a container formed by bonding flexible laminate films. For example, in a state where a recessed emboss portion that is to contain a battery electrode assembly is formed at a position where a pair of laminate films are opposite to each other, and in which the battery electrode assembly is contained in the emboss portion, outer peripheral portions of the laminate films are bonded to each other so that they are sealed. Additionally, in a configuration in which one long laminate film is doubled over to form an exterior container, an emboss portion is formed on a portion on end sides of a bent portion, and both portions are bonded to each other so that they are sealed in a state where a battery electrode assembly is contained in the emboss portion.
In the configuration in which the exterior container of the secondary battery is thus formed of flexible laminate films, there is a problem that mechanical strength is weak. Additionally, when the battery electrode assembly moves in the emboss portion, respective layers (positive electrodes, negative electrodes, separators) configuring the battery electrode assembly may be damaged so that they become deformed, and an electrical short-circuit between the positive electrodes and the negative electrodes may be caused.
Patent Document 1 discloses a configuration in which the longitudinal dimensions of the separators of the rectangular battery electrode assembly are larger than the inner longitudinal dimension of the emboss portion (rectangular containing portion) containing the battery electrode assembly, and the transverse dimensions of the separators are larger than the inner transverse dimension of the emboss portion. In this configuration, portions that protrude outside the emboss portion of the outer peripheral portions of the separators are curved or bent so that they are contained in the emboss portions.